1. Field of the Invention
The present invention relates to an exhaust gas cleaner provided with a particulate filter for collecting particulates contained in exhaust gas of an internal combustion engine and, more specifically, to a particulate filter structure capable of restricting the quick combustion of particles when the particulate filter is regenerated.
2. Description of the Related Art
Various devices have been proposed as environmental countermeasures for reducing the amount of particulates (particle material; hereinafter referred to as PM) exhausted from a diesel engine. Typically, a system has been known in which a particulate filter, such as one coated with catalyst on its surface, is provided in an exhaust pipe to collect PM. A particulate filter has a number of cells as gas passages and is adapted to adsorb and collect PM when exhaust gas passes through porous partition walls separating many such cells from each other. An oxidation catalyst may be coated on the surface of the particulate filter. As the pressure loss of the particulate filter increases when an amount of deposited PM increases, to result in an inconvenience such as the lowering of engine output, the regeneration treatment for burning and removing the collected PM is intermittently carried out so that the particulate filter is continuously usable.
A main method for regenerating a particulate filter is such that, for the purpose of burning PM, the temperature of the particulate filter is increased by controlling the exhaust gas flowing into the particulate filter to be a high temperature gas or by generating heat in the exhaust gas containing much green (unburnt) fuel by a catalytic reaction. Also, it is effective that a large amount of PM is deposited in the particulate filter and removed at once in view of minimizing the deterioration of fuel consumption due to the regeneration treatment.
However, if PM is excessively deposited, there is a problem in that PM quickly burns in a chain reaction during the regeneration treatment or when the engine is abruptly decelerated in high speed driving. That is, as the exhaust gas entering from the upstream side end of the particulate filter flows to the downstream side end thereof while gradually absorbing the combustion heat of PM when PM deposited on the particulate filter burns, the temperature in the interior of the particulate filter is usually higher toward the downstream. At this time, when the flow rate of the exhaust gas is suddenly lowered, for example, due to the abrupt deceleration, it is not possible to absorb the combustion heat of PM. Accordingly, the temperature of the downstream part of the particulate filter in a hot state rises further to cause the self-combustion of PM, resulting in a rapid chain reaction.
The rapid combustion of PM described above causes the deterioration of catalyst and, in an extreme case, the thermal destruction (cracking or melting) of a matrix of the particulate filter results. In this regard, for example, Japanese Unexamined Patent Publication (Kokai) No. 61-138812 discloses a countermeasure in which the thermal destruction of the particulate filter is avoided by shifting passage-closure positions of respective cells at the downstream end of the filter in the gas-flowing direction. Also, Japanese Unexamined Patent Publication (Kokai) Nos. 8-281034 and 05-133217 discloses another countermeasure in which the resistance to heat shock is improved by causing a thickness of a closure at each of opposite ends of the filter to be uneven or to be thicker toward the outer circumference.
The countermeasures described in these patent documents, however, are to shift the positions at which PM is deposited, so that portions which may be hot due to the combustion or portions at which the concentration of stress or combustion heat concentration may occur due to heat shock are widely distributed to avoid the generation of cracking or melting. That is, according to these countermeasures, as the temperature rise of the particulate filter itself is not prevented, the risk of damage caused by the high temperature is not eliminated when the rapid chain reaction of PM combustion occurs due to the operation condition. Also, there is another problem in that, when the particulate filter is coated with a catalyst, a catalyst may be deteriorated.
In the prior art, to prevent the particulate filter from damage, an allowable limit amount of PM to be deposited is set at a low level and the particulate filter is frequently regenerated, which causes a serious problem in that the fuel consumption is deteriorated. Thus, it has been desired to develop a technique capable of depositing a large amount of PM so that the deterioration of fuel consumption due to the regeneration of the particulate filter is avoidable.
Accordingly, an object of the present invention is to provide an exhaust gas cleaner capable of suppressing the temperature rise in the particulate filter even if the rapid chain reaction of PM combustion occurs due to the abrupt deceleration of an engine to avoid the deterioration of catalyst and the damage of the particulate filter, and capable of depositing a large amount of PM in the particulate filter and regenerating the same at once to prevent the fuel consumption of the engine from deteriorating.